With the spread of the information communication equipment such as the radio terminal, or the like, a frequency used in the communication is spreading increasingly over the broad band from several hundreds MHz for the cellular phone, or the like to several GHz for the wireless LAN, or the like. In the existing circumstances, the terminals corresponding to various communication systems respectively are used independently. In the future, the implementation of the radio terminal that is able to conform to various communication systems solely is expected.
Also, in the wake of downsizing of the radio terminal, a miniaturization of passive parts such as the filter, and the like built in the case of the radio terminal is requested. Nowadays there exists the problem that a miniaturization of the filter is difficult, particularly a miniaturization of the filter utilizing an electric resonance caused by an LC resonance circuit, or the like used often in radio communication is difficult because its resonator size depends on an electrical length. Now the filter looks for the new signal selecting principle.
In such circumstances, the development of RF-MEMS filter fabricated by the MEMS (Micro Electro Mechanical Systems) technology is advanced swiftly. This RF-MEMS filter is an electromechanical filter using a mechanical vibration of a microvibrator. In this filter, an electrical oscillation of a high-frequency signal can be converted into the mechanical vibration of the microvibrator and then this mechanical vibration is converted again into the electrical oscillation to output an output signal. Therefore, as the advantage of this filter, it can be considered that its resonator size does not depend on an electrical length and thus a size reduction of the filter can be attained. Also, this filter can be built in an RF-IC because such filter can be manufactured by the process having a good affinity for the RF-IC. Thus, this RF-MEMS filter is expected as the technology that contributes largely to a size reduction of the radio portion.
As the electromechanical filter using the microvibrator in the GHz band, the filter using a silicon substrate has been proposed (Non-Patent Literature 1, for example). In this Non-Patent Literature 1, a circular-disk type microvibrator is constructed on the silicon substrate and then a band-pass filter whose center frequency is 1.14 GHz is obtained by utilizing a mechanical resonance of the microvibrator. A signal filtering mechanism is that an electrostatic force is generated between a driving electrode and the microvibrator by a high-frequency signal being input from a signal input port to the driving electrode, and the microvibrator is excited at a frequency of the high-frequency signal. When a signal having a mechanical self-resonance frequency of the microvibrator is input, the microvibrator is largely excited and a change of electrostatic capacity is caused by a change in distance between the microvibrator and a sensing electrode. Then, the mechanical vibration of the microvibrator is picked up by the sensing electrode as the electrical oscillation because a voltage VP is applied to the microvibrator, and a signal is output from the sensing electrode to the signal output port. Namely, only the signal of a frequency that is set by the self-resonance frequency of the microvibrator can be selectively output.
Currently, an increase of an adaptive frequency and an increase of a Q value (Quality Factor) are being tried in a small-sized GHz band electromechanical filter. In order to attain a higher adaptive frequency, the self-resonance frequency of the microvibrator must be increased higher. For that purpose, a method of reducing a size of the microvibrator, a method of using a higher mode of the microvibrator, and the like are considered. In the case where a size of the microvibrator is to be reduced, when a miniaturization proceeds from micrometer order to nanometer order, amplitude of the vibration is reduced in the Angstrom order and comes close to a noise level of the quantum vibration or the thermal vibration. Therefore, the implementation of an ultrahigh-sensitivity vibration sensing method that makes it possible to measure a vibrational displacement at the quantum limit is needed.
Patent Literature 1: JP-T-10-512046
Non-Patent Literature 1: J. Wang, et al., IEEE RFIC Symp., 8-10 June, pp. 335-338, 2003